The arrival of a baby is supposed to be a happy time for a couple — so what happens when it all goes terribly wrong?

That’s the question addressed in the film “Return to Zero,” which stars Minnie Driver (“About a Boy”) and Paul Adelstein (“Scandal”) as an affluent couple expecting their first child — who tragically discover their son has died in the womb.

Premiering Saturday at 8 p.m. on Lifetime, “Zero” is the first movie to address the taboo topic of stillbirth, which was both the appeal and the challenge for Driver, who calls the role “the hardest acting job I’ve ever had in my life.”

“There was an added dimension that you’re telling a story that nobody wants to tell and that there is great merit in that,” she tells The Post.

The stillbirth takes place in the first half hour of the film — the rest focuses on the aftermath as the couple, Maggie and Aaron, struggles to cope with their postpartum grief through denial, alcohol and infidelity.

(They eventually have another child and patch things up.)

“Stillbirth is the central tragedy of the movie, but what the movie’s really about is how these people who had some cracks in their relationship, it gets blown apart,” Adelstein says. “It’s really a story about whether they’re going to be able to find each other again and forgive themselves, and forgive one another and move forward or not.”

“Return to Zero” is based on the personal experience of writer/director Sean Hanish, who wanted to shed light on an issue that is fairly common — there are 26,000 stillbirths a year in the US — but rarely talked about.

“He was very forthcoming and open about what he went through, what his wife went through as a family and as a couple,” Adelstein says. “But … he created characters — it’s not him and his wife in the script, so I think that gave him enough distance to guide us and let us be these characters and tell this story.” The film has turned into a beacon of support for those affected by stillbirth loss, inspiring a book called “Three Minus One,” and many survivors have offered their thanks on the “Zero” Facebook page.

“There is something clearly very comforting to a lot of these families to be able to reach out and say ‘This is my story,’ ” Driver says. “It’s somehow validating their children that were lost.”

Driver, who is also a musician, was even moved to write and record the title track for the movie, “Forget the Fall,” with her co-star Adelstein (he plays piano, she sings).